Tanks with a floating roof and a stillpipe are generally intended for storing relatively large volumes of petroleum products. The stillpipe is provided with a number of holes or slits along its length intended to insure that the level of the product is the same on the inside of the stillpipe as it is on the outside in the tank. The gauging of the level of the product was formerly performed by manual measuring with a measuring tape or with a float. Presently the gauging is generally performed with microwaves (radar) such as for instance in U.S. Pat. No. 4,641,139.
Present day environmental laws in numerous countries now impose rigid limits for pollution from substances that are harmful to the environment. This necessitates that tanks of different types must be designed in such a way that the outflow of volatile substances, such as by evaporation, must be limited.